1. Field of the Invention
The invention relates to an optical scanner incorporated into a laser printer or the like that records an image by an electrophotographic process, that is capable of reading image information from a document.
2. Description of Related Art
A known image reading optical apparatus capable of reading an image from the entire surface of a document obtains image information about the image by illuminating the surface of the document by, for example, a fluorescent lamp, guiding light reflected from the surface of the document by a reducing projection lens to, for example, a line sensor comprising a charge-coupled device (CCD), obtaining image information about one line of the image from the output of the line sensor, and scanning the surface of the document in a feed direction perpendicular to a scanning direction to obtain image information about the entire image.
A known optical image writing apparatus scans a photosensitive layer formed on the surface of a photosensitive drum in a scanning direction parallel to the axis of the photosensitive drum by deflecting a laser beam with a polygonal rotating mirror to form a latent image of an image on the photosensitive layer. The latent image formed by the optical image writing apparatus is developed into a toner image by a developing unit using an electrophotographic method, and the toner image is transferred to a recording sheet by a transfer unit, and the toner image is fixed to the recording sheet by a fixing unit to record the image on the recording sheet.
A prior art apparatus disclosed in Japanese Unexamined Patent Publication No. 7-199097 has both the foregoing image reading function and the foregoing image writing function, and uses optical parts common to an optical image reading unit and an optical image writing unit.
This apparatus disclosed in Japanese Unexamined Patent Publication No. 7-199097 detects light reflected from the scanned surface of a document to read an image formed on the document. Therefore, erroneous image information about the image is obtained or, at worst, there is the possibility that the apparatus is unable to read the image at all when disturbing light leaks into the apparatus.
Generally, the reflected light is detected by a photoelectric device and a photocurrent proportional to a quantity of light fallen on the photoelectric device and provided by the photoelectric device is amplified by an amplifier. The frequency of the photocurrent generated by the photoelectric device is at a minimum when the entire surface of the document is blank. The frequency is at a maximum when there are black dots or white dots on the surface of the document corresponding to a maximum resolution dependent on the diameter of a spot formed on the document by the scanning beam, the document being scanned continuously with the scanning beam deflected by the polygonal rotating mirror. Therefore, the amplifier must have a flat gain in a wide frequency range from the minimum to the maximum frequency. Such an amplifier has a large scale and hence it is impossible to construct the apparatus at a low cost. Moreover, when the operation of the amplifier is disturbed by noise, such as electromagnetic waves, the image information is affected by the noise and, consequently, an image having dark lines or blank spots is formed.